The fundamental principal of power amplification using switching technologies has been referred to as class-D, switching power amplification, digital power amplification or PWM power amplification. Class D amplifiers, in contrast to class A, class B, or class AB amplifiers, use the switching modes of transistors to regulate power delivery. The amplifiers, therefore, attain high power efficiency (i.e., low energy loss). Analog-input class-D amplifiers, however, require high precision analog components and generally require complicated analog control loops that are difficult to design. For example, an analog integrator in an approach requires a high gain amplifier, analog circuits with high swing, and high input-output linearity. The ramp generator in the same approach is difficult to design, parasitic prone, and also requires large swing of the output. Another approach only accepts analog inputs, requires a test signal to measure the frequency characteristic of the control loop. The control loop of another approach does not have high gain, and, as a result, cannot achieve high linearity. Some approaches may require complex 2nd order loops and filters, and quite a number of amplifiers, comparators, etc.
Digital input class-D amplifiers generally require a high precision DAC to convert digital data to analog. An approach uses complicated high-gain amplifiers and analog comparators, and also faces analog control loop instabilities.
Like reference symbols in the various drawings indicate like elements.